High-pressure crystal structure prediction of calcium borohydride using density functional theory

Three high-pressure polymorphs of the hydrogen storage material calcium borohydride [Ca(BH 4) 2] are predicted from structural analogs of the high-pressure titanium dioxide (TiO 2) polymorphs baddeleyite, columbite, and cotunnite. Their phase stabilities with respect to the three experimentally characterized polymorphs α, β, and γ are determined by accurate density functional theory calculations of their thermodynamic equations of state. Although these structures have not yet been observed experimentally, it is found that the proposed Ca(BH 4) 2 polymorphs are thermodynamically stable (at 0 K) at pressures above 3.6 GPa and that the ordering of the phase stabilities with respect to pressure is similar to TiO 2. © 2011 American Physical Society